High geothermal gradients and heat flow (35°C/km and 120 mW/m2) exist in Cornwall in response to abnormally high heat production (HHP) throughout a 12x03C615 km thick Hercynian post-tectonic granite batholith emplaced in the upper crust. Experiments, funded by the UK Department of Energy, are currently being undertaken by the Camborne School of Mines using boreholes 2.5 km deep with a view to establishing the feasibility of exploiting the geothermal energy resource of the granite at depths between 5 and 7 kilometres. The in situ stress field governs the response of fractured crystalline rock to hydraulic stimulation, and is an important factor in geothermal exploitation of hot, dry racks (HDR). Downward extrapolation of the measured in situ stresses using a Brace-Kohlstedt (1980) type model indicates a possible brittle/ductile transition within the Cornubian granite batholith at about 6 km depth. Support is lent to this interpretation by the distribution of hypocentral locations of natural seismic events within the granite and by preliminary interpretations of magnetotelluric soundings. The thermal effect of the HHP granite batholith extends downwards well below the base of the batholith itself, and affects the temperature distribution on the MOHO at about 28 km depth. The thermal anomaly at this depth has an important bearing on the long term elastic strength of the lithosphere underneath the batholith.
A thermal model of the lithosphere under SW England has been derived from gravity, seismic, heat flow and radiogeologic studies. This model will be used to estimate the limiting stress envelope with depth by extrapolating frictional constraints until their intersection with creep laws after the manner of Brace and Kohlstedt (1980). Vertical integration of the stress envelopes for different strain rates gives an estimate of lithospheric strength. Temperature variations at depth, caused by a large HHP granite batholith, control the strength of the lithosphere. These strength variations are tentatively linked to the tectonic and mineralisation history of the region. The weakening of the lower crust and mantle lithosphere is likely to be related to the development of the deep Plymouth Bay Basin (Day, 1986), some 10's of kilometres to the south.
The in situ rock stresses have been well determined at several sites on the Carnmenellis cupola down to depths of 2.6 km, reported by CSM (1986b), Batchelor and Pine (1986) and Hudson and Cooling (1988). These reveal a downwardly increasing stress deviator, with a minimum principal stress at 2.6 km of about half lithostatic, directed horizontal at approximately 220°. This pattern of stress with depth is very close to that expected for an ubiquitously fractured rock mass with sliding controlled by friction angles close to 40° under hydrostatic pore pressure conditions.
The E-W spine of the Cornubian peninsula is occupied by a large granite batholith (Figure 2). The batholith was emplaced at high crustal levels at the end of the Variscan orogeny (285 m y) into an Upper Palaeozoic basinal sequence thickened by folding and thrusting.
